IMPROVED RF COIL FOR INSIDE-OUT NMR/MRI SYSTEMS

Abstract

A system for NMR/MRI, having X, Y, Z directions, includes an RF coil having a B.sub.0 static magnetic field in the Z direction and a transverse B.sub.1 RF magnetic field in the XY directions. Currents in the RF coil are distributed so that the transverse B.sub.1 field is substantially uniform in the XY plane.

Claims

1. A system for NMR/MRI having X, Y, Z directions, comprising: an RF coil having a B.sub.0 static magnetic field in the Z direction and a transverse B.sub.1 RF magnetic field in the XY directions, wherein currents in said RF coil are distributed so that the transverse B.sub.1 field is substantially uniform in the XY plane.

2. The system according to claim 1, wherein a volume of interest of said RF coil lies substantially outside said RF coil.

3. The system according to claim 1 wherein the currents that generate the RF magnetic field consist of substantially parallel segments, perpendicular to said static magnetic field.

4. The system according to claim 1 wherein the direction of the current in each segment is selected to optimize the B.sub.1 field profile.

5. The system according to claim 1 wherein the uniformity of the transverse B.sub.1 field along the Z axis is optimized for uniformity along the Z axis as well.

6. The system according to claim 1 wherein a volume of interest is well defined in the X, Y and Z planes by at least 80% of total received signal.

7. The system according to claim 6 wherein the volume of interest is optimized so as to receive as uniform B.sub.1xy field as possible.

8. The system according to claim 6 wherein the volume of interest is optimized so as to receive the maximal B1xy field possible.

9. The system according to claim 6 wherein the number of lines in each layer is variable.

10. The system according to claim 6 wherein the number of layers is variable.

11. The system according to claim 6 wherein the distance between layers is variable.

12. The system according to claim 6 wherein the distance between lines in each layer is variable.

13. The system according to claim 6 wherein the dimension (width, length or thickness) of each line in each layer is variable.

14. The system according to claim 6 wherein the material of each line in each layer is variable.

15. The system according to claim 6 wherein the current direction of each line in each layer is variable.

16. The system according to claim 6 wherein the material of the subtract containing the lines in each layer is variable.

17. The system according to claim 6 wherein the coil is cooled using a cooling device such as thermoelectric cooling device, liquid nitrogen or helium.

18. The system according to claim 6 wherein the plane of the coil is rotated away from being perpendicular to the static magnetic field.

19. The system according to claim 6 wherein the coil is in a vacuum state.

20. The system according to claim 6 wherein the coil is part of a multi-coil array.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0026] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0027] FIG. 1 is a simplified graphical illustration of current for a single-layer spiral RF coil of the prior art;

[0028] FIG. 2 is a simplified graphical illustration of magnitude of B.sub.1xy vs X and Y for Z=0.5 mm (i.e., in the plane 0.5 mm above the plane of the spiral) for the prior art spiral RF coil of FIG. 1, wherein the field was obtained from the current using a Biot-Savart simulation;

[0029] FIG. 3 is a simplified graphical illustration of a magnetic field generated by a single straight conductor, showing current in a single line along Y for X=0;

[0030] FIG. 4 is a simplified graphical illustration of the B.sub.1 field produced by this single line of current;

[0031] FIG. 5 is a simplified graphical illustration of current for 6 lines of current along the Y axis (note that the total X extent of the lines is small), in accordance with a non-limiting embodiment of the present invention;

[0032] FIG. 6 is a simplified graphical illustration of B.sub.1xy for 6 lines of current of FIG. 5;

[0033] FIG. 7 is a simplified graphical illustration of current for a simple lines coil (12 lines), without return lines, in accordance with a non-limiting embodiment of the present invention;

[0034] FIG. 8 is a simplified graphical illustration of B.sub.1 field map vs X and Y for the single layer lines coil, at Z=0.4 mm above the coil surface;

[0035] FIG. 9 is a simplified graphical illustration of current for a three layer lines coil (without return lines), in accordance with a non-limiting embodiment of the present invention;

[0036] FIG. 10 is a simplified graphical illustration of B.sub.1xy for the multi-layer lines coil;

[0037] FIG. 11A is a simplified graphical illustration of single (left side of FIG. 11A) and triple layer (right side of FIG. 11A) lines coils, in which the B.sub.1 field of the triple layer coil is much larger than that of the single layer coil, in accordance with a non-limiting embodiment of the present invention;

[0038] FIG. 11B is a simplified graphical illustration of the profiles along Y-axis of the single (lower curve in FIG. 11B) and multi-layer (upper curve in FIG. 11B) lines coils, in which the profile of the B.sub.1 field along the Y axis is larger, more square and with a sharper edge for the multi-layer coil as compared with the single layer coil.

[0039] FIG. 12 is a simplified graphical illustration of |B.sub.1xy| for an eight layer spiral, in accordance with a non-limiting embodiment of the present invention; and

[0040] FIG. 13 is a simplified graphical illustration of |B.sub.1xy| for a 3 layer lines coil, in accordance with a non-limiting embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0041] In order to understand principles of the invention, reference is first made to the magnetic field generated by a single straight conductor as seen in FIGS. 3 and 4.

[0042] FIG. 3 shows a finite straight conductor along the Y axis at X=0 (with no return path) and FIG. 4 shows the B.sub.1 field this current produces.

[0043] This B1 field has a number of advantages; most importantly it is substantially uniform and well-defined, without holes or significant dips. The shape and dimensions of the B.sub.1 field can be controlled by varying the length of current conductor.

[0044] The invention may employ a field of a number of parallel conductors, which have multiple, nearly parallel lines of current, which widens the area of the B.sub.1 field. FIGS. 5 and 6 show the current and B.sub.1xy field for a configuration of 6 parallel lines of current along the Y axis at different X positions, including complete return paths below. (The extent of the lines in the X direction is only [0.4,0.4] mm.) Note that the field-of-view is fairly rectangular and much more uniform that the field-of-view for the spiral coil. In addition, there is no hole/dip in the center of the field of view.

[0045] Since the B.sub.1 field produced by a set of parallel lines has many attractive features for an inside-out NMR.MRI system, the inventors optimized the parameters of the linesthe number of lines, length of each line, inter-line distance, the number of layers, the conductivity for each line and the direction of the current (independently for each line). This is referred to as a lines coil. For each set of parameters, the B.sub.1 field was calculated using an electromagnetic simulation and various figures of merit were calculated, using calculations well-known to those skilled in the art of RF coil design for NMR/MRI.

[0046] Single Layer Lines Coil:

[0047] FIGS. 7 and 8 show the current and B.sub.1 field for an implementation of the invention for a single-layer lines coil.

[0048] Multi-Layer Lines Coils:

[0049] If the coil's resistance is not critical, one can add multiple layers. The additional layers can be tailored to accomplish a number of aims, such as but not limited to, increasing the field per unit current (B.sub.1/I), and/or improving the profile of the B.sub.1 field, adding and subtracting (i.e., cancelling) field where needed to sharpen and flatten the profile. The field may be added or subtracted by setting the direction of the current in the segment being added.

[0050] FIGS. 9 and 10 show the current for a more complex three-layer lines coil.

[0051] Comparison of the B.sub.1 Field of Single and Multi-Layer Lines Coils:

[0052] FIGS. 11A and 11B show the profiles of the field of view for the single and triple layer lines coils.

[0053] Note that the profile of the B.sub.1 field along the Y axis is larger, more square and with a sharper edge for the multi-layer coil as compared with the single layer coil.

[0054] The Z Falloff:

[0055] Until now the description examines the X-Y dependence of B.sub.1xy. The Z dependence of B.sub.1xy is also of interest. It is of course expected from basic principles of electromagnetism that B.sub.1xy falls off with Z. For the purpose of an inside-out system, which attempts to probe a specific range, ideally B.sub.1xy should be as uniform as possible within that Z range and to fall off as rapidly as possible outside that range (e.g. for Z >Z.sub.max).

[0056] FIGS. 12 and 13 show |B.sub.1xy| in the Y-Z plane (note the difference in the Z scale for the two plots). Note that the line coil has a lower B.sub.1/I but a better (i.e. deeper) Z penetration.

[0057] It is noted that the direction of the current in each line segment determines the direction of the B.sub.1xy field it produces. Thus by adding lines and/or layers one can either increase or decrease the B.sub.1yx field depending on the direction of the current in each segment. In addition, by controlling the conductivity of each line one can control the current it produces and hence the B.sub.1xy field it creates.